1. Field of the Invention
The present invention relates to a method of controlling a waste water purification plant wherein two or more parameters, such as the concentration of ammonium, nitrate, oxygen and phosphate and oxygen supply, are measured and the plant is automatically controlled on the basis of the measurement values obtained using a mathematical model for the purification process.
2. The Prior Art
When treating, e.g., municipal waste water in a biological purification plant, where a removal of nitrogen and phosphate-containing compounds and organic matter is typically desired, considerable variations occur during a day, both in the concentration of the polluting substances in the waste water and in the amount of waste water supplied to the purification plant. The great variation in the load of the plant makes heavy demands on the control of the plant in order to prevent the concentration of the polluting substances in the effluent from exceeding the maximum limit values.
The control of biological purification plants is typically based on measurements of primarily phosphate, nitrate, ammonium and oxygen carried out with photo or electrochemical electrodes (sensor systems). Such sensor systems are very sensitive and require continuous maintenance and frequent calibration, in particular in connection with measurements carried out in liquid mixtures from biological purification plants, which i.a. is due to the fact that such liquid mixtures contain many interfering substances, and that the particles and active sludge contained therein give rise to bacterial growth in the sensor systems as well as clogging thereof.
As a result, measurements carried out using sensors, and in particular the measurements of nitrate and ammonium, are very unreliable and present a central problem in the control of biological purification plants.
"Computer Control of an Alternating Activated Sludge Process" by Kummel M. and Nielsen M. K., published at The International Symposium on Process Systems Engineering, Kyoto, Aug. 23-27, 1982 discloses a method of controlling a biological purification plant comprising two treatment tanks which are alternately operated in anoxic and aerobic conditions and wherein the flow pattern is changed accordingly and so that the untreated waste water is supplied to the anoxic tank, from which it is carried to the aerobic tank and therefrom further on in the plant to a clarification tank in which a sedimentation of active sludge is carried out, which sludge is subsequently recycled in the plant for introduction into the anoxic tank, and from which clarification tank the effluent is discharged.
The control is effected by means of a computer collecting the measurement results, analyzing the results on the basis of a mathematical model, and implementing new control strategies.
In the noted prior art method, measurements of oxygen, ammonium and nitrate are carried out using suitable sensors, the control parameters used being the oxygen supply rate and the nitrification and denitrification period ratio.
In the prior art method, the measurements of ammonium and nitrate concentrations are used continuously to determine the corresponding optimum oxygen concentration (the set point) during the nitrification and denitrification processes, respectively.
Furthermore, the nitrification and denitrification period ratio is controlled relative to the content of ammonium in the untreated waste water, i.e., such that the nitrification period is prolonged when the ammonium load is high and shortened when the ammonium load is low, and vice versa for the denitrification period.
"Real Time Optimization of a Sewer System as Part of a Larger Municipal Prototype Project Including Treatment Plant Operation", by Lindberg S. et al., Fifth International Conference on Urban Storm Drainage, Osaka 1990, pp. 1299-1303, discloses a system for controlling a sewer and waste water treatment plant.
This control system comprises a data quality control module serving to evaluate the quality of the measurements carried out of the hydraulics of the plant. The evaluation is effected by discarding measurements which, in the light of the retrospective course of the measurements, are considered to be erroneous or which deviate considerably from what is to be expected on the basis of other hydraulic measurements.
The quality evaluation is effected by discarding measurement values which are not comprised within a value interval having fixed minimum and maximum limits and/or by discarding values, the changes of which compared to the most recent measurement are outside a value change interval having fixed minimum and maximum limits.
EP-A-0,446,036 discloses an apparatus for controlling a system, e.g., a waste water purification plant, the apparatus comprising 1) a number of measuring units, 2) means for analysing measurement data in order to select a characteristic data set, 3) means for analysing the characteristic data set .in order to identify a possible operation problem, 4) means for analysing the operation problem in order to find a strategy for resolution of the problem, and 5) means for controlling the system on the basis of the strategy.
The means for analysing measurement data is used i.a. for affecting an evaluation of measurement data. The evaluation step does not include the use of a mathematical model of the deterministic/stochastic type but is based merely on the use of functions expressing empirical a priori information about the time course of the parameter for which a measurement value is being evaluated.